Flexographic printing plates are well known for use in letterpress printing, particularly on surfaces that are soft and easily deformable, such as packaging materials, for example cardboard, wrapping paper and plastic films. Flexographic printing plates can be prepared using photopolymerizable compositions, such as those described in U.S. Pat. No. 4,323,637 (Chen et al) to form relief images. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable solvent (such as a developer) removes the unexposed areas of the photopolymerizable layer, leaving a relief image that can be used for flexographic printing.
Imagewise exposure of a photosensitive element conventionally requires the use of a phototool which is a mask, having transparent and opaque areas covering the photopolymerizable layer. The mask prevents exposure and polymerization in the opaque areas. It allows exposure to radiation in the transparent areas so that the underlying layer polymerizes and remains on the support after the development step. The mask is usually a photographic negative of the desired printing image. If corrections are needed in the final image a new negative must be made. This is a time-consuming process. In addition, the mask may experience slight dimensional changes due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may provide different results and could cause registration problems.
Thus, it would be desirable to avoid the use of such masks and to directly record information on a photosensitive element, for example by means of a laser beam. The image to be developed could then be translated into digital information and the digital information used to direct the laser for imaging. The digital information could even be transmitted from a distant location. Corrections could be made easily and quickly by adjusting the digitized image. In addition, the digitized image could be either positive or negative, eliminating the need to have both positive-working and negative-working photosensitive materials, or positive and negative masks. This would save storage space and, thus, reduce cost. Another advantage would be that registration can be precisely controlled by a machine during the imaging step. Digitized imaging without a mask would be particularly well-suited for making seamless, continuous printing forms.
In general, it has not been very practical to use lasers to directly image the elements that are used to prepare flexographic printing plates. Such elements have low photosensitivity and require long exposure times even with high powered lasers. In addition, most of the photopolymerizable materials used in these elements have their greatest sensitivity in the ultraviolet region of the electromagnetic spectrum. While UV lasers are known, economical and reliable UV lasers with high power are generally not available. However, non-UV lasers are available which are relatively inexpensive, and which have a useful power output and can be used to form a mask image on top of flexographic printing elements.
U.S. Pat. No. 5,262,275 (Fan) describes a photosensitive flexographic printing plates having a laser ablatable masking layer. This layer is capable of absorbing infrared radiation but is opaque to actinic radiation, coated over a barrier layer and photopolymerizable layer. During use, the masking layer is imagewise ablated using IR radiation, forming a mask image that blocks actinic radiation in the areas of the photopolymerizable layer where development is desired. The element is then overall exposed with actinic radiation to cure the exposed areas of the photopolymerizable layer, followed by processing in a suitable solvent (or developer) to remove the unexposed areas of the element Thus, a flexible relief image in a final flexographic printing plate is produced.
Ablation techniques have a disadvantage in that they produce solid debris that can be a hazard and requires wiping and collection to insure that it does not materially affect the desired image. U.S. Pat. No. 5,705,310 (Van Zoeren) describes an element similar to that described in the Fan patent, but it also has a cover sheet for collecting material ablated from the mask layer during the imaging step.
The elements and methods for their use described in these references also suffer from the disadvantage that ablation is a binary process, meaning that it produces only either opaque or essentially transparent areas upon imaging, and does not provide areas of intermediate density. In other words, a mask image formed using ablation tends not to have continuous tone images.
Besides ablation processes, laser imaging is known for a number of other well-known applications such as color proofing and dye transfer. As described in EP 0 679 531A1 (Savini et al), laser-induced transfer processes may be used in dye sublimation processes to transfer colorants to receiver elements. However, these processes are discouraged because of resulting inferior images that are mottled and have other image defects.
There is a need in the industry to have flexographic imaging elements that provide excellent continuous tone images and that can be prepared without the use of ablation.